1. Field of the invention
The present invention relates, in general, to air conditioning apparatus. More specifically, the invention relates to an air conditioning apparatus including a variable capacity compressor. The capacity of the compressor is changed through an inverter device in response to changes of an air conditioning load within a certain space.
2. Description of the prior art
Air conditioning apparatus including a variable capacity compressor, an inverter device and a control circuit, such as, e.g., a microcomputer, can vary its cooling/heating capacity by varying the speed of the compressor in response to changes in the air conditioning load.
The microcomputer outputs a digital signal to the inverter device when the microcomputer detects the change of the actual temperature with reference to a predetermined set value temperature.
The frequency of the AC voltage produced from the inverter device is varied on the basis of the digital signal from the microcomputer. The AC voltage from the inverter device is fed to the drive motor of the compressor, and the rotational speed of the drive motor is then varied in accordance with the AC voltage. The cooling or heating capacity of the air conditioning apparatus depends on the rotational speed of the compressor.
FIG. 1 is a graph illustrating the relationship between the output frequency and the output voltage of the inverter device of the air conditioning apparatus, as described above.
As can be seen in FIG. 1, the output voltage of the inverter device rises on a straight line path from the low voltage Vmi to the high voltage Vma between the minimum frequency Mi Hz and the maximum frequency Ma Hz. A large torque is generated within this range. Therefore, the motor of the compressor can be driven in a stable condition by the torque described above. On the other hand, the torque is smaller between the starting frequency St Hz at which the motor of the compressor begins to rotate and the minimum frequency Mi Hz. This is because the output voltage of inverter device between Mi Hz and St Hz is lower in comparison with the voltage between St Hz and Ma Hz.
The starting control operation of a conventional air conditioning apparatus will be described with reference to FIG. 2.
A control device composed of a microcomputer calculates a target frequency Ta Hz between the minimum frequency Mi Hz and the maximum frequency Ma Hz on the basis of the difference between a desired room temperature and a present room temperature. The control device outputs a digital starting signal to an inverter device. Based on the stating signal, the inverter device feeds the motor of the compressor with an output which gradually rises from the starting frequency St Hz to the target frequency Ta Hz. If the inverter device outputs the target frequency Ta Hz immediately after receiving the starting signal, a large amount of the starting current is needed.
During winter, since the compressor is usually installed outside house, the viscosity of the lubricating oil in the compressor is high. Furthermore, the refrigerant of the refrigerating cycle is in liquid form and is accumulated undre the lubricating oil because of its larger specific gravity.
Therefore, the friction between the rotating or sliding mechanical parts of the compressor is high, and thus the starting load is increased.
Under this state, if the invertor device provides the output, as shown in FIG. 2, to the motor of the compressor, the compressor often fails to start because of the small starting torque and the high frictional resistance, as described above.